Positron emission tomography (PET) is a type of nuclear imaging that is used in a number of applications, especially medical diagnostic and research imaging. In a typical state-of-the-art positron emission tomography system, one type of radioactive compound, such as fluorodeoxyglueose (FDG) which is a radiopharmaceutical, is administered to a patient or other living organism under surveillance. Positrons, which are positively charged particles, are emitted by the isotopes of the radioactive compound as the isotopes decay within the body. Upon emission, tthe positron encounters an electron, and both are annihilated. As a result of one annihilation, gamma rays are generated in the form of two photons. It was discovered approximately thirty years ago that these two photons are emitted in approximate opposite directions (about 180 degrees) from one another. The precise position of the positron-emitting isotope can be determined by surveying these photons. Traditionally, PET scanners accumulate information concerning the lines of travel of the emitted photons at different angles around the body under surveillance, and process this information through a computer to generate a tomographic image of the distrbution and concentration of the isotope. In this connection, the PET scanner can observe and quantify biochemical and physiological changes that occur naturally and in disorders in the human body.
Traditionally, positron emission tomography systems employ discrete scintillators, usually bismuth germanate crystals arranged in rings. Typically there are approximately one hundred or more detectors per ring, with up to five rings in the detector structure. A coincidence event is an event in which gamma rays are given off across the axis of the subject, defined as a line along which the positron annihilation must have occurred. Spatial resolution in current PET scanning systems is limited by the detector resolution capability. In addition, current PET scanning equipment is complex and very expensive to manufacture and to maintain. Further, state-of-the-art position encoders employ analog techniques to detect and analyze the photon emissions. Such analog techniques are unstable and can vary with the detector gains used in analyzing the signals.
Accordingly, it is an object of the present invention to provide a two dimensional photon counting position encoder system and process having improved spatial resolution characteristics.
Another object of the present invention is to provide such a system and process which are significantly less expensive to manufacture and maintain.
It is yet another object of the present invention to provide a photon detector which includes a slotted or tuned light guide to facilitate the encoding process.
Still a further object of the present invention is to employ a technique and means for counting the photons rather than using less stable analog position processing techniques during the accumulation of information for generation of the tomographic image. Further, pattern recognition techniques are used in one embodiment to decode detector gathered information.